This invention relates to thermosettable resin compositions and their preparation. In a specific embodiment, the invention relates to bismaleimide/bisbenzocyclobutene copolymers having improved toughness, reduced melt viscosity and improved viscosity stability.
Advanced composites are high-performance materials made up of a fiber-reinforced thermoplastic or thermosettable material. Thermosettable materials useful in advanced composites applications must meet a set of demanding property requirements. For example, such materials should optimally have good high-temperature properties such as high (above 200.degree. C.) cured glass transition temperature and low (less than 3%) water absorption at elevated temperature. Such materials should also exhibit superior mechanical strength, as reflected in measurements of Mode I fracture toughness above 2 MPa.m.sup.1/2. For ease of processing in preparing prepregs for composite parts, the uncured material ideally has a low (below 120.degree. C.) melting temperature.
Examples of thermosettable materials useful in advanced composites include epoxy resins, bisbenzocyclobutene resins and bismaleimide resins. Epoxy resins have good processing properties, but generally have relatively low glass transition temperatures and unacceptable high-temperature water absorption, and they are also generally brittle.
Standard homopolymers of bisbenzocyclobutene resins (as described in the examples of U.S. Pat. No. 4,540,763) are brittle unless very high molecular weight resins are used. If very high molecular weight bisbenzocyclobutene resins are used, however, the materials are difficult to process into fiber-reinforced composites because of their high viscosity. Bismaleimide resins have this same disadvantage of brittleness and further tend to have high melting points and must be used with solvents in order to be readily processable. In addition, cured bismaleimide resins tend to have high (in the 5-7% range) water absorption. Copolymers of bisbenzocyclobutenes and bismaleimides as illustrated in the Example of U.S. Pat. No. 4,730,030 have improved properties including heat resistance but, like the cured homopolymers, lack the necessary toughness for high-performance applications. Bisbenzocyclobutene/bismaleimide uncured mixtures also have a tendency to exhibit viscosity instability in the molten state.
It is therefore an object of the invention to provide bisbenzocyclobutene/bismaleimide mixtures having improved viscosity stability. It is a further object of the invention to provide bisbenzocyclobutene/ bismaleimide copolymers having greater toughness.